1. Field of the Invention
The present invention refers to a process for the preparation and modification of additives, based on zeolite with a high silica-alumina ratio (SAR), such as ZSM-5 (Zeolite Socony Mobile In. 5) capable of increasing the production of LPG and of propene in low-severity operations in fluid catalytic cracking (FCC) units, in order to maximize the production of medium-distillate fractions with low aromaticity.
The present invention provides a method for obtaining modified additives, which minimizes molecular cracking in an LCO range and makes an increase in gasoline octane possible in low-severity operations.
2. Description of the Related Art
Fluid catalytic cracking (FCC) is performed by the contact of hydrocarbons with a fine particulate matter catalyst in a pipe reaction zone or riser. Loads that are commonly submitted to FCC processing are, usually, petroleum refinery process streams that come from longitudinally segmented vacuum towers, called heavy Vacuum Gas Oil (VGO), or chains that are heavier than before coming from the bottom of atmospheric towers, called Atmospheric Residue (AR), or even mixtures of these two streams. Typically, these streams, have a density in the range of 8 to 28 o API, and must be chemically processed using a process such as the catalytic cracking process, which fundamentally alters its chemical composition, converting them into lighter, and more valuable hydrocarbon streams.
From the time of its initial conception, the FCC process has been a process essentially aimed at the production of high octane gasoline, and is also responsible for the production of LPG. The medium distillates (LCO) produced by this process represent between 15% and 25% of the total yield and correspond to a distillation range that typically goes from 200° C. and 340° C. Normally, LCOs have a high concentration of aromatics, which may surpass 80% of the total composition, a fact that makes it difficult to incorporate into the diesel pool. The current and future scenario points to a slow down in the consumption of gasoline and an increase in diesel oil. With the increase in the demand for high quality medium-distillates to the detriment of the gasoline market in mind, changes in the mode of operation of FCC units have been discussed, for the purpose of increasing medium-distillate production in FCC processing. Nevertheless, the high concentration of aromatics in LCO is responsible for its high density and its abysmal explosive quality in diesel motors (low cetane number). The high level of aromatics also makes it difficult for hydrotreatment processing to improve their properties for this purpose.
For the purpose of increasing the yield of medium-distillates and at the same time reduce the level of aromatics; several works discuss modifications in the catalytic system and in the operational fluctuations in order to attain a reduction in the severity of the process. Among the operational conditions a reduction in the reaction temperature and a reduction to the catalyst-to-oil ratio (CTO) are included. The form of operation most commonly used to maximize medium-distillates in the FCC process use a reduced reaction temperature at extremely low numbers (between 450° C. and 500° C.), low activity catalyst, and minimized catalyst circulation. All these measures achieve an increase in the yield and improve the quality (by a reduction of aromatics) of the LCO produced. On the other hand, they reduce the conversion with a subsequent reduction in the production of LPG and propene, which in turn reduces the octane of the gasoline. Some important references on this issue are listed below: 1) Distillate yield from the FCC: process and catalyst changes for maximization of LCO: Catalysts Courier, R. W. Peterman; 2) Hydrocarbon Publishing Company: Advanced hydrotreating and hydrocarbon technology to produce ultra2-clean diesel fuel, 2004; 3) Studies on maximizing diesel oil production from FCC: Fifth international symposium on the advances in fluid catalytic cracking, (218th National meeting, American Chemical society, 1999); 4) New development boosts production of middle distillate from FCC: Oil and Gas Journal (August, 1970).
The industrial application of additives, based on zeolite with a high silica-alumina ratio (SAR), such as ZSM-5 (Zeolite Socony Mobile), began in 1983. Since then, ZSM-5 has been used with great success in FCC processing as an active component of the additives, to increase yield in light hydrocarbons, such as liquefied petroleum gas (LPG) and light olefins of high aggregate value, such as propene and isobutene.
It has also been observed that the use of this additive promotes an increase in octane in gasoline, accompanied with a reduction in the yield of that fraction. The literature relative to this issue may be verified in the publications: 1) F. Degnan, G. K. Chitnis, P. H. Schipper, History of ZSM-5 fluid cracking additive development at Mobil, Microporous and Mesoporous Materials, 35-36 (2000) 245-252; 2) S. P. Donnelly, S. Mizzahi, P. T, Sparrel, A. Huss, Jr., P. H. Schipper and J. A. Herbst, How ZSM-5 works in FCC, Division of Petroleum Chemistry, ACS Meeting, Aug. 30-Sep. 4, New Orleans, 1987; 3) A. S. Kishna, C. R. Hsieh, A. R. English, T. A. Picoraro, C. W. Kuehler, Additives improve CFC process, Hydrocarbon Processing, November (1991) 59-66. Notwithstanding, it uses a reaction temperature on the order of 540° C. to maximize gasoline. Under these conditions, practically no change is seen in the yield or the quality of LCO.
Studies carried out in the Petrobras Research Center have proven that the use of ZSM-5 additives in catalytic cracking processing that operate at low severity by reducing the reaction temperature, promote a reduction in LCO yield and an increase in aromatics. Thus, the presence of this commercial additive in FCC operations to maximize medium-distillates becomes harmful, since its effects are contrary to the main objective of the operation.
The present invention proposes the alternative of using ZSM-5 in soft fluid catalytic cracking (MFCC), in other words, in typical low severity fluid catalytic cracking operations. Using this innovative method this additive is modified by the depositing rare earths, in order to promote a partial blockage of the zeolite pores, making the molecular cracking difficult in the LCO range and, in turn, keeping the remaining sites active or sufficient to perform cracking of smaller molecules, in the gasoline range, guaranteeing an overall increase in light olefins.
In the specialized literature, the use of zeolite ZSM-5 is widely cited as being used in conjunction with rare earths in various applications, such as in the production of light olefins, the reclamation of CO2/CH4 to obtain synthesis gas, n-butane dehydrogenation and cracking, catalytic reduction of NOx by propene and aromatization of C6-olefin flows to BTX (benzene, toluene, and xilenes). The use of rare-earth-exchanged ultrastable Y (REUSY) zeolite is also cited in FCC to promote a greater catalytic tolerance to vanadium and to increase gasoline yield in detriment to gas yield. The use of rare-earth-exchanged ultrastable Y (REUSY) zeolite in FCC catalysts has the objective of improving the FCC catalyst's stability when in the presence of vanadium and to increase gasoline yield in detriment to gas yield. An increase in the level of rare-earth-exchanged Y zeolite promotes transfer reactions of hydrogen in the FCC process, resulting in an increase in the formation of aromatic compounds through dehydrogenation reactions of naphthenic compounds. However, this effect is contrary to the desired effect for these applications of this invention. The literature relative to the use of the exchanged rare earth in ZSM-5, may be verified in the written publications described below: 1) Zhicheng, S. et al. U.S. Pat. No. 5,380,690; 2) Zhang, W. D., et al. Preparation of La2NiO4/ZSM-5 catalyst and catalytic performance m CO2/CH4 reforming to Syngas. Applied Catalysis A: General, v. 223, p. 85-102, 2002; 3) Wakui, K. et al. Dehydrogenative cracking of n-butane using double-stage reaction. Applied Catalysis A: General, v. 230, p. 195-202, 2002; 4) Yokoyama, C. and Misono, M. Selective reduction of nitrogen monoxide by propene over cerium-doped zeolites. Catalysis Today, v. 22, n. 1, p. 59-72, 1994.
The China Petro-Chemicals Corporation possesses a patent citing the formula for a Pentasil type zeolite catalyst, modified with phosphorus and rare earths, for the production of light olefins.
The catalyst consists of a mixture of zeolite (1-50% p/p), kaolin (0-70%) and inorganic oxides (5-99% p/p). The zeolite mixture consists of rare earth exchanged zeolite Y (REY), the concentration of which ranges between 0 and 25% p/p, and between 75 and 100% p/p of a Pentasil type zeolite containing phosphorus and rare earths. However, this patent seeks to improve the catalyst activity in typical FCC processing conditions and, therefore, it does not take into account the amount nor the quality of the LCO fraction.
Wakui et al. studied pure ZSM-5 cracking of n-butane for high conversions. The authors observed that with rare earth exchanged ZSM-5 zeolites more ethene is formed in detriment to the formation of aromatic compounds. They discuss the fact that rare earths in ZSM-5 favor monomolecular cracking reactions in detriment to bimolecular reactions such as, for example, the transfer of hydrogen. Thus, this type of application seeks to crack compounds with low molecular weight, in conditions that are much more drastic than the conditions which apply to the present invention.